The invention is directed to improvements in fuel injection systems, in particular unit fuel injectors.
In a known Diesel injection system of this type (German Offenlegungsschrift 37 00 359), a high degree of freedom in terms of open and closed-loop control interventions in the control processes pertaining to injection is advantageously obtained by the use of an intermediate piston, specifically by providing that quantities of fluid are metered largely independently of one another in two separate chambers, namely the pump chamber and the pressure chamber. One of these fluid quantities, located in the pump chamber determines the supply onset, and hence, depending on the injection quantity stored, the end of injection as well; the second fuel quantity, metered into the pressure chamber, is injected as an unmodified injection quantity. This advantage has a particularly favorable effect in direct injection engines (in contrast to chamber engines), with the high injection pressures they require, where the dictated high feed pressure in the pump chamber and pressure chamber has a not inconsiderable influence on the particular fuel volume, because of the compressibility of the fuel. This change in volume would have a particularly disadvantageous effect given the oblique-edge control means predominantly used in mechanically controlled injection systems, where the quantity control is effected by deviation during the compression stroke.
In the generic injection systems here, contrarily, although a fuel quantity metered into the pressure chamber at low pressure via the metering device is also compressed during the compression stroke, nevertheless it is injected as desired, in the predetermined amount. The compressibility of the fuel cannot have a disadvantageous effect. Since the speed of the pump piston is rpm-dependent, that is, the piston speed is higher, the higher the rpm, and whenever an adequate transmission pressure has been established in the pump chamber as a result of the volume enclosed there, this speed is transmitted directly upon the motion of the intermediate piston; the result is a very short injection duration at high rpm and a corresponding longer injection duration at lower rpm, at the same load or in other words at the same injection quantity. In the short injection duration available at high rpm, a maximum quantity (full load) must necessarily be injectable, and at low rpm this is no problem because of the longer period of time available. Through the various operating ranges, with the injection quantity varying by a factor of from 1 to 30 from idling up to full load, however, this condition means that the available time during idling, which is relatively long because of the low rpm, is not utilized for an optimal injection course, given the relatively small injection quantities during idling. On the other hand, the engine noise is particularly annoying during idling, although it is well-known that lengthening the injection duration, which would theoretically be possible, particularly in the idling range, would lead to a drop in engine noise.
In a known fuel injection system ("Bosch-Distributor Injection Pump with Centrifugal Governor", Bosch Technical Instruction, Vol. 6, 1978, No. 2, FIG. 15 on page 98 and 99; see also U.S. Pat. No. 4,407,253), some of the fuel quantity pumped by the pump piston out of the pump work chamber flows out via a throttle conduit during the compression stroke and during idling, while another portion of the fuel attains injection. The injection quantity required for maintaining idling is nevertheless injected, because the mechanical governor of this injection pump compensates for the portion flowing out via the throttle conduit by correspondingly shifting the diversion of fuel from the pump work chamber occurring during the compression stroke. With this division of the supply quantity, a longer period of time is obtained for the portion to be injected than would be the case if no fluid were to flow out via the throttle conduit.
Although theoretically it would be conceivable to provide such regulation to lengthen the injection duration in the injection system of this generic type, to do so would cancel out some of the originally obtained degree of freedom. Furthermore, at very high injection pressures, the elasticity of the fuel would engender additional control errors, which would be very difficult to correct. This is true particularly for unit fuel injectors, which have a very compact pumping region (pump chamber and pressure chamber) and usually operate at very high pressures (up to 1800 bar).